Act-2 is a member of a family of small secreted proteins, many members of which have inflammatory or chemotactic activities. This family, whose members are now denoted as 'chemokines' can be divided into two subfamilies based on whether the first two of four conserved cysteines are adjacent (CC) or separated by one amino acid (CXC). Act-2 is a member of the CC subfamily, and is presumed to be the human homologue of macrophage inflammatory protein (MIP)-1beta. Act-2/hMIP-1beta is chemotactic for CD4+ T lymphocytes. Synthesis and secretion of this cytokine are rapidly induced in T cells, B cells, and monocytes following stimulation with antigen or mitogen. During the past year, we determined the three dimensional structure of Act-2/hMIP-1beta by multimensional NMR. To accomplish this project, it was necessary to produce in large quantities unlabeled, 15N-labeled, and 15N plus 13C doubly labeled Act- 2/hMIP-1beta protein. The structure is extremely interesting in that it demonstrates that the quaternary structures of CC and CXC chemokines are extremely different from each other and provides a rational for the observation that these two classes of chemokines do not bind to each other's receptors. Knowledge of the structure is vital to eventual drug agonist/antagonist development as well as being scientifically important since no structural information was heretofore available on any member of the CC subfamily.